1. Field of the Invention
The present invention is directed to a nuclear magnetic resonance tomography device of the type having a basic field magnet and a gradient coil system for the generation of magnet field gradients in three spatial directions disposed perpendicularly to one another and an antenna system for transmitting high-frequency pulses and for the reception of magnetic resonance signals from an examination volume. The invention also is directed to a method for operating a magnetic resonance tomography device.
2. Description of the Prior Art
Different magnetic fields and electromagnetic fields are employed in magnetic resonance tomography. The examination object lies in a strong magnet field of approximately 0.2 through 4 T. High-frequency fields in the range of 10 through 160 MHZ are utilized for the excitement of the nuclear spins. Magnetic field gradients, i.e., location-dependent magnet fields are superimposed onto the basic magnet field for spatial resolution. The magnet field gradients are multiply switched within a pulse sequence for the excitation and selection of the magnetic resonance signals. Magnet field gradients with a short rise time and a high intensity are necessary for images with high spatial resolution and short measuring time.
A magnetic resonance tomography device of the type mentioned above is known from German OS 40 04 184, for example.
The time-dependent magnet field gradients induce currents in conductive parts. This is true not only for metallic items built into the examination space of the magnetic resonance tomography device, but also is basically true for the examination object. Given short rise times and high amplitudes, peripheral nerve stimulations can occur in persons to be examined, these nerve stimulations mainly being manifested by muscle convulsions. The examined persons described these stimulations as convulsions in the buttock region, back region and also in the nose root, depending on the examined part of the body. Given gradient intensities of 15 mT/m with rise times up to 500 W.mu./s, aggravating stimulation problems generally do not result. When the gradient intensity and/or the switching time and further intensified physiological borders are reached, since painful stimulations can then occur. As shown by W. Irnich in the publication "Electrostimulation by time-varying magnetic fields", in MAGMA 2 (1994), 43-49, the stimulation threshold is thereby given by the critical change of the magnetic field intensity .DELTA.B. The absolute value of the magnetic field intensity change therefore determines the stimulation threshold, rather than one of the magnetic field gradient given fixed switching times.
In order to prevent peripheral muscle stimulations, the extent z.sub.max of the magnet field gradient in the longitudinal axis of the measuring object is usually limited, however, the imaging field is also limited as a result. This results in areas of the object that are desired to be imaged not being able to be shown in an examination, even when the spatial expanse of the homogeneity volume of the basic field would allow this.
To address the aforementioned stimulation problem, German OS 42 25 592 suggests to cover stimulation-sensitive areas outside the examination area with a closed conductor loop. According to this published patent application, the stimulation problem mainly exists in the direction of the longitudinal axis of the magnet in magnetic resonance tomography devices wherein the patient is surrounded by ring-shaped magnet coils (typically given superconducting magnet constructions). This direction, which coincides with the lengthwise direction of the examination object, is usually referred to as the z-direction. The technique of covering stimulation-sensitive areas outside of the examination area makes the handling of the magnetic resonance tomography device more difficult.